It is known to adjust the viscosity characteristics of oil through the use of polymeric additives such as ethylene-propylene copolymers, ethylene-propylene dienemonomer copolymers, polyisobutylene, polymethacrylate copolymers, and hydrogenated diene-styrene copolymers. It is known to form both random and block styrene-diene copolymers with the block polymers being formed by either sequential polymerization or coupling. In addition to simple difunctional coupling agents which produce a linear polymer it is known to form a coupling agent in-situ by the polymerization of a monomer having two separately polymerizable vinyl groups such as divinylbenzene. This gives a star polymer having from about 6 to as many as 50 or more arms. It is also known to couple polymer chains with tetravalent silicon compounds such as silicon tetrachloride. However, after the first two chlorine atoms are replaced by polymer chains the other two positions become increasingly more difficult to substitute and consequently, the polymers can turn out to be linear or mixtures of 3-armed radial polymers.
Consequently, styrene-diene oil additives have been made using in-situ produced coupling agents from divinylbenzene to give a large number of arms when star polymers were desired. Sequential polymerization or simple difunctional coupling agents have been used for the production of linear polymers for use in oil applications. Tetravalent silicon coupled polymers have heretofore not been considered candidates for commercial viscosity index improvers.
The viscosity of lubricating oils varies with temperature. In general, oils are identified by a viscosity index which is a function of the oil viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature are given in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C. Historically, a high viscosity index (less decrease in viscosity with increasing temperature) has been sought for commercial oil formulations. In fact, however, the viscosity index is only one indicator of oil quality. More relevant are oil characteristics at the extreme operating conditions actually encountered. Such extreme conditions include temperatures of from about -40.degree. to 150.degree. C. and high shear rates comparable to those existing in the bearings and pumps of modern engines.